Abstract

Magnetic separatrix is an important boundary layer separating the inflow and outflow regions in magnetic reconnection. In this article, we investigate the sub-structures of the separatrix region by using two-and-half dimensional electromagneticparticle-in-cell simulation. The separatrix region can be divided into two sub-regions in terms of the ion and electron frozen-in conditions. Far from the neutral sheet, ions and electrons are magnetized in magnetic fields. Approaching the neutral sheet, ion frozen-in condition is broken in a narrow region (∼c/ωpi) at the edge of a density cavity, while electrons are frozen-in to magnetic fields. In this region, electric field Ez is around zero, and the convective term –(vi × B) is balanced by the Hall term in the generalized Ohm’s law because ions carry the perpendicular current. Inside the density cavity, both ion and electron frozen-in conditions are broken. The region consists of two sub-ion or electron-scale layers, which contain intense electric fields. Formation of the two sub-layers is due to the complex electron flow pattern around the separatrix region. In the layer, Ez is balanced by a combination of Hall term and the divergence of electron pressure tensor, with the Hall term being dominant. Our preliminary simulation result shows that the separatrix region in guide field reconnection also contains two sub-regions: the inner region and the outer region. However, the inner region contains only one current layer in contrast with the case without guide field.

Received 19 April 2012Accepted 06 July 2012Published online 23 July 2012

Acknowledgments:

This work was supported by the National Science Foundation of China (NSFC) under Grants 41004060, 40890163, 41174147 and the Specialized Research Fund for State Key Laboratories, and Scientific Research Program of Education Department of Jiangxi Province under Grant GJJ11049.